Chemical vapor deposition processes for ceramic or metallic materials on a semiconductor wafer require particular chemical species to be deposited on the surface of the wafer. The chemical species are carried along with or entrained in a carrier gas. In accomplishing the task of measuring the flow rate of a certain species, for example a relatively low volatility reactive chemical species entrained in a relatively high volatility carrier gas in a conduit for a manufacturing or other process, one might ideally use the equation: ##EQU1## where P.sub.R and P.sub.C are the partial pressures of the reactant gas and carrier gas respectively, and F.sub.R and F.sub.C are the flow rates of the reactant gas and carrier gas, respectively. However, in actual practice, it has been noted that an undesirable decrease in the count of desired species often occurs over time especially where the reactive chemical is an organo-metallic species of relatively low volatility. One speculates that the decreased flow of the reactant species may be due to unintended condensation, instabilities in the delivery system, or other causes.
Prior art methods for controlling the flow rate of the reactant species measure the pressure in the delivery line of the combined delivery and reactant gas, and then cause the carrier gas flow to increase if the measured pressure in the delivery line drops. However, chemical vapor deposition (CVD) systems have relatively higher flow rates of reagents as compared to other layer formation processes. Thus, relatively higher pumping speeds are presented to the delivery system, such as a bubbler, as compared with other deposition systems. Additionally, CVD systems have reactant species that are relatively less volatile, e.g. organo-metallic compounds, and have steeper vapor pressure curves than prior art systems which use simple pressure monitoring feedback loops.
Thus, it would be desirable if a suitable monitoring apparatus and method could be devised for these latter processes.